The present invention relates to a compressor for use in a cryogenic refrigerator.
Compressors of this kind are described in Japanese Patent No. 2522424 and Japanese Patent Publications (KOKAI) No. 5-288419 and No. 8-110110. FIG. 9 is a vertical sectional view showing a compressor disclosed in Japanese Patent Publication No. 8-110110, and FIG. 10 is a vertical sectional view showing only a movable portion thereof. In FIGS. 9 and 10, a piston 3 is inserted into a cylinder having a cylindrical space, via a gap 2, to form an operating-gas-compressing space 5 in the cylindrical space of the cylinder 1 enclosed by a cylinder head 4. A piston shaft 6 is coaxially secured to the piston 3. The piston 3 is supported by two support springs 7 and 8 for free reciprocation in the axial direction. The support springs 7 and 8 are composed of plate springs attached to the piston shaft 6 with an axial gap formed therebetween.
The piston 3 is driven by a linear drive section 9 to reciprocate in the axial direction. The linear drive section 9 includes a driver coil 11 wound around a coil bobbin 10 secured to the piston shaft 6, and a magnetic circuit having a void 12 in which the driver coil 11 is accommodated. The magnetic circuit is formed by an annular magnet 13, and a flanged cylindrical yoke 14 and an annular yoke 15 arranged on the respective sides of the magnet 13. The yoke 14 has a cylindrical frame 16 connected thereto, and the frame 16 has a bottomed two-stage cylindrical frame 17 connected thereto. The cylinder head 4, the cylinder 1, the yoke 4, and the frames 16 and 17 constitute one pressure container generally forming a gas chamber 34. The gas chamber 34 leads to the compressing space 5 via the gap 2.
Here, a procedure of assembling a major portion of the compressor in FIG. 9 will be described. The yokes 14 and 15 are stuck to and integrated with the permanent magnet 13 by using an adhesive. The frame 16 is combined with the integrated parts, and the combined parts are tightened together using screws (not shown). Then, the support spring 7 is combined with the yoke 14 from above in FIG. 9 via their fitting portions, and these combined parts are tightened together by using screws (not shown). Then, the piston shaft 6, integrated with the piston 3, is inserted into a central hole in the support spring 7 from above. Furthermore, an interval tube 18, a coil bobbin 10, a washer 19, a sleeve 20, and the support spring 8 are sequentially fitted on the piston shaft 6 from below as shown in FIG. 10. At the same time, the support spring 8 is combined with the frame via their fitting portions, and the combined parts are tightened by using screws (not shown).
Further, on the piston shaft 6, the support spring 7, the interval tube 18, the coil bobbin 10, the washer 19, the sleeve 20, and the support spring 8 are tightened between the piston 3 and a washer 21 by a nut 22. Subsequently, the frame 17 is combined with the frame 16 via their fitting portions and secured thereto by fillet welding. Reference numerals 23a and 23b denote a movable portion and a fixed portion, respectively, of a displacement sensor for detecting the axial displacement of the piston 3. The movable portion 23a is attached to the piston shaft 6 after tightening the nut 22. As described above, a unit formed of the piston 3, the support springs 7 and 8, the yokes 14 and 15, the frames 16 and 17, and others, which are integrally assembled, is inserted into the cylinder 1, which is separately supported on an assembly frame. The piston 3 is carefully aligned, and the yoke 14 is then tightened against the cylinder 1 by using a screw 24.
In such a compressor, magnetic fluxes generated by the permanent magnet 13 return from the N pole surface thereof through the yoke 15, the void 12, and the yoke 14 to the S pole surface thereof. Thus, when a current is periodically conducted through the driver coil 11, a magnetic force is generated between this current and the magnetic fields in the void 12 to reciprocate the piston 3 in the axial direction, thereby compressing an operating gas in the compressing space 5. A pressure wave from the compressed gas is applied to a cryogenic refrigerator (not shown) through a gas channel 25 in the cylinder head 4.
The above conventional compressor has the following problems.
(1) The piston 3 has the piston shaft 6 secured thereto, the piston shaft 6 has the linear drive section 9 arranged radially outside the piston shaft 6, and the support springs 7 and 8 are arranged on the respective sides of the linear drive section 9 in the axial direction. Thus, the piston 3, the linear drive section 9, and the plurality of springs 7 and 8 are linearly arranged in the axial direction, resulting in a long movable portion to increase the longitudinal dimension of the compressor.
(2) Between the support springs 7 and 8, the fitting portions are present between the support spring 7 and the yoke 14, between the yoke 14 and the frame 16, and between the frame 16 and the support spring 8. Accordingly, parts and assembly errors may be accumulated in the fitting portions to cause misalignment between the support springs 7 and 8. This misalignment may incline the axis of the piston 3 to the cylinder 1 to bring the piston and the cylinder into contact with each other, thus causing friction therebetween.
(3) For assembly, the support spring 7 and the piston 3 are inserted from one side (from the upper side in FIG. 9) of the linear drive section 9 in the axial direction, and the interval tube 18, the coil bobbin 10, and the support spring 8, and others are inserted from the other side (from the lower side in FIG. 9). Consequently, one-direction assembly can not be made on the linear drive section 9, preventing an easy assembly operation.
Thus, the object of the present invention is to solve these problems by decreasing the size of compressor, increasing the accuracy thereof, and allowing the compressor to be assembled more easily.
The present invention provides a compressor comprising a cylinder having a cylindrical space, a piston inserted into the cylinder via a gap forming a clearance seal, the piston forming an operating-gas-compressing space in the cylindrical space, support springs composed of a plate spring for supporting the piston for free reciprocation in the axial direction, a linear drive section for driving the piston to reciprocate in the axial direction, and a pressure container that forms a gas chamber leading to the compressing space via the gap. The linear drive section is formed by a driver coil connected to the piston, and a magnetic circuit composed of a permanent magnet having a void in which the driver coil is located. The spring is arranged at an interval at the end of the corresponding piston which is opposite to the compressing space, and the linear drive section is arranged radially outside the compressing-space-side end of the piston (a first aspect of the invention).
According to the first aspect of the invention, one end of the piston is supported by the plurality of support springs in a cantilever manner, and the linear drive section is arranged radially outside the other end of the piston. Accordingly, the entire length of movable portion can be shorter than that of the piston, so that the longitudinal dimension of the compressor can be decreased. Further, since the plurality of support springs is arranged together at one side of the linear drive section, a surface of the compressor main body on which the support springs are fitted can be shared easily by the support springs. Consequently, the support springs can be more accurately aligned with each other, and the plurality of support springs can be assembled on the linear drive section from one direction.
In this case, the driver coil, permanent magnet, and yoke of the linear drive section are preferably arranged in the radial direction of the piston. This arrangement reduces the axial dimension of the linear drive section (a second aspect of the invention).
The cylinder and the yoke are preferably integrated. This arrangement eliminates an assembly error between the cylinder and the yoke, and reduces the number of parts required (a third aspect of the invention).
Further, the yoke and the main body of the pressure container are integrated, and the main body preferably supports the support springs. This arrangement eliminates an assembly error between the pressure container main body and the yoke, and reduces the number of parts required. Further, one-way assembly is enabled in which the coil bobbin and the support springs are inserted over the piston from the same side of the axial direction, and the piston is also inserted into the cylinder from this side (a fourth aspect of the invention).
On the other hand, the support springs are preferably installed so that a front or rear side of one of the support springs faces a rear or front side of the other, respectively. It is difficult to completely offset different spring characteristics of the front and rear sides of the support spring. However, by arranging the front or rear side of one of the plurality of support springs so as to face the rear or front side of the other, respectively, the above different characteristics can be offset to prevent a tip of the piston from swinging during reciprocation, as well as the rotation of the piston caused by the torsion of the support springs can be prevented (a fifth aspect of the invention).
A proper value for the interval between the support springs is determined by the structural analysis based on the weight of the movable portion (including the piston, the coil bobbin, the driver coil, and others), the rigidity of the support springs, and the gap between the piston and the cylinder. To maintain this proper interval, an interval piece is preferably provided to define the interval between the support springs (a sixth aspect of the invention).
Preferably, a lubricating solid coat, which can be detached whenever necessary, of a thickness corresponding to the gap is applied to one or both of the inner peripheral surface of the cylinder and the outer peripheral surface of the piston, and the inner peripheral surface and the outer peripheral surface are fitted to allow the piston to be inserted into the cylinder (a seventh aspect of the invention). By inserting the piston into the cylinder by the fitting, the piston and the cylinder can be accurately aligned with each other (a seventh aspect of the invention).
The compressor can be constructed such that a pair of the pistons face each other with the compressing space being located therebetween. The compressing space is shared by these pistons. Thus, vibrations caused by the reciprocation of the pistons can be offset, whereby the vibration of the entire compressor can be minimized (an eighth aspect of the invention).
Furthermore, in the above compressor, a driver coil on the piston side is formed with electricity by an external connection terminal fixed to the pressure container, via a lead. Since the piston reciprocates in the axial direction relative to the pressure container, a driver-coil-feeding lead is constructed to move in the axial direction, and one end thereof is held on the piston side, while the other end thereof is held on the pressure container side. In the present invention, this driver-coil-feeding lead is provided between the support springs (a ninth aspect of the invention). This allows the driver-coil-feeding lead to be arranged in a space in which the support springs are accommodated, thus reducing the longitudinal dimension of the compressor compared to the case in which this lead is arranged axially outside the support spring.
In the compressor according to the ninth aspect of the invention, an intermediate terminal joining the driver-coil-feeding lead and the driver coil, and another intermediate terminal joining the driver-coil-feeding lead and an external connection terminal are provided on the piston side and the pressure container side, respectively, by penetrating support springs (a tenth aspect of the invention). This arrangement eliminates the need to pass the wires connecting the driver-coil-feeding lead and the driver coil, and the wires connecting the driver-coil-feeding lead and the external connection terminal through the holes in the support springs, thereby simplifying a wiring operation. Moreover, if the driver-coil-feeding lead and the internal terminals are integrally coupled, no wire is required which connects the driver-coil-feeding lead and each of the intermediate terminals, thereby further simplifying the wiring operation (an eleventh aspect of the invention).